This invention relates to furnaces for heating glass articles, in particular glass sheets, of the type wherein the glass is subjected to blasts of hot gas and/or supported on a layer of hot gas. Such furnaces are typically employed to heat the glass in preparation for tempering, bending, or other treatment. Examples of gas-support furnaces for heating glass sheets may be seen in U.S. Pat. No. 3,223,501 to Fredley et al. and in U.S. Pat. No. 3,332,759 to McMaster et al.
It is customary with such furnaces to utilize a blower to generate the pressure required to blast the hot gases against the glass, and it is preferred to recirculate the hot gases from the furnace back through the blower in order to conserve energy. But providing such a recirculating blower is usually a problem. If the blower is located within the furnace enclosure itself, it must be made of costly heat resistant materials, but even then the severe operating conditions often lead to excessive wear and frequent maintenance problems. In an attempt to overcome this problem, blowers have been placed outside the furnace, and the hot gases conducted to and from the furnace through conduits. However, this approach does not entirely avoid the problem because operating conditions for the blowers are still quite severe, and withdrawing the gases from the furnace causes disadvantageous heat losses. Moreover, the energy consumption of such a blower is relatively heavy regardless of location, and thus it would be desirable to provide pressurized air to a glass treating furnace by more efficient means.
One alternative to the use of a blower is disclosed in U.S. Pat. No. 3,607,173 to McMaster et al. An external source of compressed air is used to draw furnace gases into a plurality of venturi devices termed "inspirators." While that arrangement eliminates the need for a hot gas blower, it is apparent that it represents fewer economies in energy usage than would be desired since the large number of inspirators shown would require a large amount of compressed air. Even more significant is the difficulty which such an arrangement presents in heating the incoming compressed air without harmfully detracting from the amount of heat imparted to the top surface of the glass. This is because providing sufficient heat transfer area for the large number of compressed air lines in front of the gas burners could obstruct the transfer of heat onto the top surface of the glass. Furthermore, the transfer of heat through the compressed air pipes is a limiting factor.
Glass heating furnaces of the gas-support type usually employ the combustion of gaseous fuels, usually natural gas, as their energy source. In view of the recent shortages and occasional, localized curtailment of natural gas supplies, as well as the rising costs of gaseous fuels, it would be desirable to provide this type of furnace with the capability of using electrical energy as its sole or major heat source, but to do so has heretofore been considered too inefficient.